Volume diffusion as a mechanism for discordant lead ages

Abstract

The discordant U-Pb ages of sixteen minerals having Pb207/Pb206 ages of 2400 to 2700 m.y. closely fit a 2800 to 600 m.y. chord when placed on a pb207/U235-Pb206/U238 diagram. These minerals are from the continents of North America, Africa, Europe, Australia, and Asia. If the data are interpreted as evidence of an episode of loss of lead 600 m.y. ago from minerals that were crystallized 2800 m.y. ago, it is strange that the same time of loss is indicated for all four continents. Moreover, most of the samples are from shield areas where no evidence has been found for metamorphic events 600 m.y. ago. As an alternative explanation, lead may be considered to diffuse continuously from crystals at a rate governed by a diffusion coefficient (D), the effective radius (a), and the concentration gradient. Calculations of the present-day Pb206/U238 and Pb207/U235ratios as a function of the parameter D/a2 yield ratios that lie on a 2800 to 600 m.y. chord on a Pb206/U238-Pb207/U235 diagram for losses of up to two-thirds of the lead produced in the sample. This method is capable of explaining the regularity of the age patterns from one continent to another and does away with the need for episodic losses of lead simultaneously in all the areas 600 m.y. ago. Other examples have been found of suites of minerals with discordant ages which fit this hypothesis; they include minerals with apparent ages of 1900, 1700, and 1100 m.y. The behavior of uranium-lead ages in several zircons from metamorphic rocks is indicative of an activation energy for diffusion of lead of less than 10 kcal/mol over a temperature range of 50° to 400°–600°C. This inference, when considered with available data for the diffusion of argon from micas, is capable of explaining why uranium-lead ages measured from zircon may be either greater or less than the potassium-argon ages of coexisting mica.